For anyone with a taste for such things, the Monster of Florence remains the most fascinating serial killer of the 20th century. In many ways, the events read like a novel: there's a killer with a gruesome "signature"; there's a link to a murder from years ago; there are endless sexual perversions; there's the backdrop of a stunning city which itself becomes a character. The only thing that stops it being a perfect crime novel is that there's no resolution, no neat Poirot wrap-up. Even today, nobody knows who the Monster of Florence was.

Douglas Preston was already an established crime writer when, in 2000, he moved his family to Florence. He intended to write a novel set in the city and contacted a veteran journalist for advice about the workings of the criminal justice system. By chance, the man he called on was the "monsterologist" Mario Spezi. Spezi quickly realised Preston was living right next to one of the Monster's murder scenes. Inevitably, Preston promptly abandoned the novel and decided to write about the Monster instead.

Much of the story they tell is already well known. In 1974, a couple making love in a car were murdered. The girl's breasts and pubic area had been stabbed 97 times. Then, in the 1980s, the Monster returned. He struck twice in 1981, then again in 1982, 83, 84 and 85. Each time, the victims were making love in a car and on each occasion the Monster removed the vagina, and once the breast, of his female victims. The murder weapon was always the same Beretta .22.

It was the same weapon, police realised, used in another double murder: in 1968, another couple making love in a car had also been murdered. The only problem was the perpetrator had been found and had confessed. He'd been in prison during the subsequent murders.

Various men were accused, imprisoned and then released. The hysteria during the 1980s was such that police were overwhelmed with tip-offs and false leads. Anyone with a grudge was pointing the finger. Other murders, some completely unrelated, some possibly linked, became part of the investigation. The result was that Florence began to appear less a centre of culture and more a city of dark secrets. Investigators were led down dead ends which nevertheless revealed something about Florence: there was the priest who enjoyed shaving the pubic hair of prostitutes; the man who forced his wife into orgies; there were the indiani, the Italian voyeurs, who, with sophisticated equipment, watched young couples making out in their cars.

One of those voyeurs, Pietro Pacciani, was convicted in the 1990s of being the Monster. He was acquitted on appeal. But by then the investigation had become a monster itself. Such was the high-profile nature of the case that people became ever more desperate for a conviction, and ever more devious in the ways they would obtain it. It is probable that evidence was planted. A conspiracy theorist and various simpletons with remarkable memories became key witnesses.

By the early part of this century, a new investigator, Michele Giuttari, seemed almost deranged: he was hunting down a satanic sect that used the sex organs as "wafers" in black masses. Anyone who got in his way was assumed to be part of the conspiracy.

The second part of the book is less about the Monster and more about this new investigation. Spezi and Preston wrote articles and published this book in Italy, rubbishing the new line of inquiry. Spezi was thrown into prison. Preston was indicted and warned to leave Italy. An old friend of Spezi's was arrested and is currently standing trial, the latest in a long line of improbable Monsters. This friend felt, he said, "like someone who has fallen into a film, knowing nothing of the plot or characters".

There have been many books published on the subject before. Various investigators, journalists, protagonists and novelists (including Magdalen Nabb and, tangentially, Thomas Harris, whose Hannibal Lecter was at least loosely inspired by the events in Florence), even Spezi himself, have all tried to capture the hypnotic horror of the Monster in the past. But this new book is one of the best I have read. The combination of Spezi's expertise with Preston's narrative skills and his outsider's clarity of vision makes it both detailed and clear. They perfectly evoke the contrast between the gruesome crimes and the sublime backdrop.

Towards the end, amid the rising paranoia of being themselves under investigation, Spezi and Preston even manage to suggest who they believe is the real Monster of Florence. As in the best novels, it's a character who's been before your eyes all along, but who, because he seemed too young or too wounded, you ignored. Whether or not their suggestion is true, it makes for a satisfying end to a gripping read.